The present invention relates to a high temperature composite material having particular utility in jet engine applications and a method for forming said composite material.
Gas turbine engine performance will increasingly depend on the temperature capabilities of newer and novel structural materials. In the turbine section of the engine, these materials will likely have to sustain gas path temperatures in excess of 2100xc2x0 C. A significant number of components in the turbine section, as well as in other engine sections, will typically be formed from fiber reinforced composite materials. In the future, successful composite materials will have to be substantially pore free, and processed under less aggressive thermal environment in order to avoid damage to the reinforcing fibers which will typically be incorporated into these composite materials.
Currently, composite materials under evaluation for engine components are ceramic, intermetallic, and metal matrix composites reinforced with continuous fibers. Typically, they are fabricated using methods that call for powder metallurgical processing techniques such as cold pressing followed by sintering and hot pressing. Three of the problems encountered during such processing are:
1. At the consolidation temperatures which are typically high, there is significant thermal damage to the fiber due to high temperature exposure. As a consequence, the residual strength of the fiber decreases considerably;
2. The reactivity between the fiber and the matrix increases due to high processing temperature. This results in chemical degradation of fiber and in the formation of strong bonds; and
3. The space enclosed between neighboring fibers does not get as fully penetrated by the matrix powder as the rest of the sample. As a consequence, the interfiber space exhibits more porosity than elsewhere. The shape and the size of the powder particles play an important role in this problem.
The emerging technology of nanocrystalline materials provides a potentially attractive and viable solution to these problems. Nanocrystalline materials are polycrystals with crystallite sizes typically between 1 and 100 nanometers. When consolidated, these materials exhibit a structure in which as high as 50% of the solid consists of incoherent interfaces between crystals which are differently oriented. Since grain boundaries occupy such a large volume fraction of the structure, nanocrystalline materials exhibit some properties which are radically different from those of their crystalline counterpart. These properties include diffusivity, plastic deformation and alloying capability. The diffusivity in materials in nanocrystalline form is several orders of magnitude faster than in the corresponding crystalline counterpart. A very attractive feature of nanocrystalline materials is the capability to introduce alloying in systems which ordinarily do not exhibit any mutual solid solubility.
A variety of different techniques for manufacturing nanocrystalline structures and for forming components therefrom are known in the art. U.S. Pat. No. 4,909,840 to Schlump, for example, illustrates a process of manufacturing nanocrystalline powders and molded bodies from the powders. The Schlump process of manufacture involves providing a number of components in powdered from and mixing the components in elementary form or as pre-alloys. The components have particle sizes ranging from 2 to 250 microns. The power components are then subjected to high mechanical forces in order to produce secondary powders having a nanocrystalline structure. The secondary powders are then processed into molded bodies according to known compression molding processes, but at a temperature below the recrystallization temperature.
U.S. Pat. No. 5,147,446 to Pechenik et al. illustrates a method for fabricating dense compacts from nano-sized particles using high pressures and cryogenic temperatures. The Pechenik et al. method comprises pre-compacting or agglomerating at starting powder having a particle size of from 1.0 to 50 nanometers into particles of 50 to 100 microns in size and a density of from 20% to 40% of true density; placing the pre-compacted powder into a pressure cell; lowering the temperature of the cell below room temperature; and applying pressure from 0.1 to 5 GPa to the cell while maintaining the cell at the lowered temperature, thereby forming the compact.
U.S. Pat. No. 5,123,935 to Kanamaru et al. relates to the manufacture of Al2O3 composites used for machining tools for difficult-cutting steels. The strength and toughness of the Al2O3 composites are increased by dispersing a titanium compound with an average grain size of not greater than 0.1 microns into Al2O3 particles with an average grain size of not greater than 1 micron. SiC whiskers are uniformly dispersed between each of the Al2O3 particles to form a nano composite structure. For this purpose, the SiC whiskers containing 0.3% to 1.5% by weight of oxygen, dissolved uniformly in a solvent, are mixed with a starting Al2O3 series powder containing Ti compounds to form a uniform mixture, which is then molded and sintered.
As yet, no one has addressed the problems of forming composite materials for jet engine components which will encounter gas path temperatures with improved fiber strength retention after processing conditions in excess of 2100xc2x0 C. as well as the problem of filling the voids which exist between reinforcing fibers used to provide such composite materials with strength and toughness.
Accordingly, it is an object of the present invention to provide composite materials having utility in high temperature environments.
It is a further object of the present invention to provide composite materials as above formed from nano-sized powders.
It is still a further object of the present invention to provide an improved method for forming such composite materials which allows the composite materials to be densified at temperatures significantly lower than what is called for by conventional crystalline counterparts and thereby avoid thermal and chemical damage to reinforcing fibers embedded therein.
Still other objects and advantages of the present invention are set out and will become clear from the following description.
The foregoing objects are attained by the composite materials and the method of the present invention.
Composite materials in accordance with the present invention comprise a matrix formed from a powdered material having a particle size in the range of from about 1 to 100 nanometers, preferably from about 4 to about 5 nanometers, and a plurality of reinforcing fibers embedded within the matrix. The reinforcing fibers comprise from about 20% to 40% by volume of the composite material. The composite materials of the present invention are further characterized by the substantial absence of voids between adjacent ones of the fibers.
The matrix may be formed from materials selected from the group consisting of refractory oxides with ionic bonds, non-oxides such as nitrides and carbides with covalent bonds intermetallic compounds, metals and alloys. The reinforcing fibers are selected from the group consisting of single crystal fibers, polycrystalline single fibers, multi-filament tows of Al2O3 fibers, silicon carbide fibers, and silicon nitride fibers having a diameter less than or equal to about 10 microns and a length equal to or greater than about 5 microns.
The composite materials of the present invention may be manufactured by mixing the powdered nano-sized material and the fibers and consolidating the mixture of powdered ceramic materials and fibers. Consolidation can be performed using a hot pressing technique or, alternatively, a cold pressing technique and sintering at elevated temperature. If the latter approach is used, a hot isostatic pressing step can be performed after sintering at atmospheric temperature for final consolidation.
Other details of the composite materials of the present invention and the method of manufacturing them are set out in the following detailed description.
As previously discussed, the present invention relates to high temperature composite materials having utility in the manufacture of jet engine components.
The composite materials of the present invention are characterized by a matrix formed from nano-sized powdered materials and a plurality of reinforcing fibers embedded within the matrix material. While there is a wide variety of materials which could be used for the matrix of the present invention, it is preferred to use refractory oxides with ionic bonds such as Al2O3 and ZrO2, which may be fully or partially stabilized, carbides and nitrides with covalent bonds such as silicon carbide and silicon nitride, and intermetallics such as molybdenum disilicides. The nano-sized powdered material used for the matrix may be formed using any suitable technique known in the art. The matrix material particles however should have a size i the range of from about 1 to about 100 nanometers, preferably from about 4 to about 5 nanometers.
The composite materials of the present invention are further characterized by the presence of reinforcing fibers to improve the strength, creep resistance and toughness of the composite material. The fibers are typically present in a concentration of about 20% to about 40% by volume. It has been found that fewer fibers results in an insufficient amount of support and that too many fibers cause unwanted contact between fibers in the final composite material.
Preferably, the reinforcement fibers will be continuous and thermodynamically stable in the chosen matrix. The fibers should also have a thermal expansion coefficient not significantly different from that of the matrix material. Other important properties for the selection of reinforcing fibers are strength, modulus, diameter and length. Preferably, the fibers have a diameter equal to or less than about 10 microns and a length greater than or equal to about 5 microns. Fibers are distinguishable from whisker type materials by virtue of their aspect ratios. Whiskers have aspect ratios of several tens to several hundreds; whereas fibers have much larger aspect ratios.
Suitable fibers for incorporation into the composite materials of the present invention include single crystal and polycrystalline single filament as well as multi-filament tows of Al2O3, SiC fibers such as NICALON, both as weave and CVD monofilament, and silicon nitride fibers. For example, single crystal alumina (sapphire) fibers may be used in the composite materials of the present invention.
While it is preferred to use continuous fibers in the composite materials of the present invention, it is also possible to use chopped fibers so long as they meet the foregoing diameter and length requirements. Fibers below 5 microns in length and a micron or less in diameter are considered to be hazardous to health and therefore, should be used with caution.
Fabrication of the composites of the present invention involves several steps. First, if the reinforcing fibers are to be continuous fibers and to be oriented in a particular direction, then the fibers must be wound on a drum for unidirectional orientation or weaved for more complicated two dimensional arrangements. Thereafter, the matrix material is delivered to the fiber. This can be done by either mechanical transport of the matrix material or by using a liquid transport vehicle such as alcohol, water or some other suitable dispersing agent and subsequently eliminating the liquid transport vehicle. When a liquid transport agent is used, the resulting slurry may have any desired viscosity. Once the matrix is applied, the materials need to be dried. The final step of the method comprises consolidating the mixture of the powdered matrix material and the reinforcing fibers. This can be done by elevated temperature sintering in vacuum or inert environments, hot pressing to form a compact, or a combination of pressing to form a compact, sintering and/or hot isostatic pressing. The consolidating step may be carried out using any suitable pressure and residence time at temperature. The temperatures which can be used are matrix material dependent and are discussed hereinafter. The following example of provided to illustrate the manner in which a composite material in accordance with the present invention may be fabricated.